The long-term goal is to understand mechanisms of synaptic functional and structural plasticity in brain. This is important because such plasticity represents cellular mechanisms enabling memory. Matrix metalloproteinases (MMPs) are a family of extracellular peptidases whose targets include extracellular matrix (ECM). Canonically, they function to remodel the pericellular microenvironment. Here, we address the novel hypothesis that in brain, regulated MMP-mediated extracellular proteolysis coordinates synaptic signaling and remodeling during synaptic and behavioral plasticity. In Aim 1, regulation and activation of MMP-9 during synaptic plasticity is studied in vivo using electrical stimulation protocols and field recordings to elicit different forms of synaptic plasticity in area CA1. Pharmacological methods are used to establish the timecourse over which levels of MMP-9 protein and proteolytic activity are regulated;gain- and loss-of- function approaches are used to establish functional roles of MMP-9 in synaptic physiology and plasticity;novel reagents and methods including the use of fluorescently tagged MMP active-site directed probes are used to determine if MMP-9 is activated globally during plasticity or locally in relationship to plastic synapses. In Aim 2, effects of active MMP-9 on neuronal form and function are tested. Gain- and loss-of- function approaches combined with two-photon time-lapse imaging of living dendritic spines and whole-cell recording will be applied to acute hippocampal slices to: a) characterize changes in spine motility, morphology and actin dynamics in relationship to MMP-mediated potentiation;and b) determine the relationship between MMP-9 mediated structural plasticity and integrin activation. In Aim 3, a hippocampal- dependent learning and memory task will be used in conjunction with subsequent biochemical, anatomical and loss-of-function approaches in order to determine: a) the timecourse and localization of learning-induced MMP-9 activation;and b) the effects of neutralizing proteolytic activity on strength of memory. The experiments will reveal new, fundamental roles for MMPs in normal brain function, and provide new insight into molecular mechanisms that regulate synaptic and behavioral plasticity.